FR2464462A1 - METHOD AND DEVICE FOR MEASURING THE LEVEL OF A LIQUID IN AN ENCLOSURE - Google Patents

Abstract

METHOD OF MEASURING THE LEVEL OF THE SURFACE OF A LIQUID IN A VERTICAL AXIS ENCLOSURE. THIS PROCESS IS CHARACTERIZED IN THAT WE PLACE ON A SAME VERTICAL NEAR THE WALL OF THE SAID ENCLOSURE N SOURCES OF RADIATION CAPABLE OF PRODUCING RADIATION BEAMS IN THE SAID ENCLOSURE, AS WE PLACE ON A SECOND VERTICAL CONTAINED IN THE CONTAINED PLAN OF THE SAID AXIS AND OF THE SAID FIRST VERTICAL N RADIATION DETECTORS NN AVAILABLE AT DIFFERENT LEVELS, IN WHICH THE SAID DETECTORS ARE MEASURED THE INTENSITY OF THE RADIATION HAVING CROSSED THE SAID SPEAKER AND AS DEDUCTED FROM THE DIFFERENT MEASUREMENTS THE LEVEL SURFACE OF THE LIQUID OR THE LOCAL DENSITY OF THE DIPHASIC MIXTURE. APPLICATION TO THE MEASUREMENT OF THE LEVEL OF THE LIQUID WATER CONTAINED IN THE PRESSURIZER OF A PRESSURIZED WATER NUCLEAR REACTOR.

Description

The present invention relates to a method and a measuring device

the level of a liquid content

in an enclosure.

  More specifically, the present invention

  identifies a method and a device for measuring the level of water in a liquid contained in an enclosure, this liquid being

in equilibrium with its vapor.

  Even more precisely, the invention is particularly applicable, but not exclusively to the

  measuring the level of the liquid water contained in the press

  riser of a PWR nuclear reactor.

  It is known that in nuclear reactors of the type

  PWR, the maintenance of pressure in the primary circuit

  The pressure of the reactor (which is in the reactor vessel) of about 155 bar is provided by a pressurizer which is constituted by a closed chamber in which the pressure of the primary circuit is maintained by equilibrium between the liquid phase and the reactor. gaseous phase of the water. A depressurization accident being particularly serious

  it is especially interesting to control the function-

  of this pressurizer and therefore the level of the

liquid in this one.

  The measurement of the liquid level in the press

  riser according to the method which will be described later

  to provide complementary and redundant information

  particularly useful in certain conditions of

  reactor operation (large transient

  of incident ...). This process makes it possible to calibrate and

  complete the current measurements by pressure difference

  sometimes posing problems of representativeness (

  ve, hysteresis, registration, inability to detect any

two-phase situations).

  A first object of the invention is to measure,

  by interaction with radii the level of a

  quide in a chamber, this liquid being surmounted by a gas.

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  A second object of the invention is to measure the level of a pressurized and hot liquid in equilibrium with its vapor in an enclosure, this measurement not only making it possible to determine a level of equilibrium between the vapor and the liquid, for example water, but also to give an image of the zones of two-phase equilibrium

  the vapor and the liquid and the estimate of the density cor-

  sponding. These are essentially liquid areas in which there are vapor bubbles and, on the other hand, essentially vapor zones in

  which are water droplets.

  A third object of the invention is such a measurement by interaction of radiation with the liquid, more particularly applied to the case of pressurizers for pressurized water nuclear reactor, that is to say, the case

  o the water may possibly have some

primary life.

  A fourth object of the invention is to perform this measurement in the case of the pressurizer of a reactor

  pressurized water supply taking into account disturbances

  measurements due to the corrosion of the walls

  the chamber containing the vapor-liquid mixture of water.

  The invention in general terms consists in arranging according to a first generator of the enclosure N transmitters of

  collimated radiation whose beam is directed

  according to the diameters of the enclosure, and to arrange

  a generator diametrically opposed to the first genera-

  detector to collect the radiation that has passed through the chamber containing the liquid-gas mixture. The attenuation of the radiation is thus measured and it is possible, by identification of the axial map of the counting rates of the detectors, to measure the position of the liquid level. Moreover, the device having been calibrated during its implementation (initial measurement: empty enclosure, then with different water levels), it can be estimated, at several vertical dimensions,

  the average water density along the radiation path.

  Preferably, the number of radiation sources

  N is much lower than the number N 'of radiation detectors.

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  According to a first mode of implementation, the radiation is a radiation produced there by a source

radioactive material such as cobalt 60.

  According to a second embodiment, the radiation is a neutron beam produced, for example,

  by a 14 MeV neutron source obtained by reacting

deuterium-tritium.

  In any case, the invention will be better understood

  upon reading the following description of several modes

  implementation of the process and the implementation of the

  corresponding examples given by way of example

non-limiting.

  The description refers to the appended figures

  on which is represented: in FIG. 1, a vertical sectional view of a pressurizer showing the implantation of the sources of

  radiation and implantation of radiation detectors.

  is lying; - In Figure 2, a horizontal sectional view showing in more detail an embodiment in which the radiation is a radiation y; - In Figure 3, a side view of an embodiment of the collimator associated with the detector; FIGS. 4 and 5 show diagrams illustrating a mode of detection of the level by means of radiation sources y and detectors, and in FIG. 6 an example of a two-phase configuration inside the pressurizer illustrating a of the

  advantages of the method which is the subject of the invention.

  In Figure 1, there is shown an example of implementation of the method. In this figure, we see in vertical axial section the pressurizer 2 consists of a

  cylindrical side wall 2a and closed by a bottom

  2b. In this chamber, there is a liquid phase 6 in equilibrium with a vapor phase 8. It has been symbolized by

  the horizontal line 10 the interface between these two environments ..

  Of course, we know that in fact this separation can not

not be so rigorous.

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  To ensure level detection, we have

  along a line parallel to a generatrix of the side wall 2a radiation sources S, S2 and S3. According to a straight line parallel to a generatrix of the pressurizer side wall 2a and contained in the same axial plane as the first generator, a plurality of

  radiation detectors with the general reference D1.

  These detectors cover partially or totally the

axial height of the pressurizer.

  In Figure 2, there is shown in more detail a part of this installation in the case where the radiation source is a source y. There is of course the side casing 2a of the pressurizer 2. For example, the internal diameter of the pressurizer is 213.4 cm and the wall thickness is 10.8 cm. The source S, which is for example a source of cobalt 60 of 1000 curies is

  housed in a shield 20 which is extended by a collima-

  22 to the wall 2a. This collimation is designed to limit the radiation of one of the sources S, to a portion of space around the vertical plane containing the source and the generator on which the detectors are aligned. For example, the distance between the contact with the wall 2a and

the source is 100 cm.

  Arranged on the same diameter of the pressurizer,

  find a detector D1. This detector can be a scintilla-

  sodium iodide. This detector is also housed in a shield 22. Between the detector D1 and the wall 2a and inside the shield 22, there is preferably a collimator 24 formed of metal plates parallel to the incident radiation and whose section can be by

  example in the form of a grid. This grid is constituted

  killed by a set of metal sheets. In Figure 3, we better see this collimator which has for example a pitch of 1 cm. For example, the length of the collimator according to

  the path of the radiation is 120 cm.

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  We know that radiation passing through a material

  given material has a certain attenuation rate (T) in a straight line. By placing a detector in front of the source we can measure the radiation that has passed through the liquid or gas and deduce the density of this liquid or

  of this gas or even of the liquid-gas mixture.

  By arranging sources and detectors to

  different levels in the axial direction of the pressuri-

  In this way, it is possible to draw an axial map of the density of the fluid filling the pressurizer. However, it must be remembered that the radiation must also pass through the metal walls of the pressurizer. The table below gives the average free path for the radiation y according to

his energy.

TABLE I

  Free average runs for shock-free flow calculations for gammas of 1 MeV P H20 liquid = 593.2 kg m3 155 bar P H20 vapor = 102.4 kg m-3 345 C Particles XFer XH20 liquid.H20 vapor Gammas of 1 , 33MeV 2,46 cm 27,53 cm 159,47 cm Gammas of 1,17MeV 2,31 cm 25,76 cm 149,22 cm Gammas of 800 KeV 1,92 cm 21,45 cm 124,24 cm If the we consider the particular example illustrated in Figure 2 and the numerical data mentioned above, we would have the following counting rates: TV 6x104 s-1 in vapor phase, TL 90 s-1 in liquid phase

  for a pressure of 155 bar and a temperature of 345 C.

  In these calculations, it is considered that the detector Dl is placed

  in front of the source S1 and at the same height.

  As indicated, it is necessary to take into account the deposited activity corresponding to the

  corrosion on the walls of the pressurizer. However, the systems

  Gridded collimation systems shown in Figure 2 minimize the influence of deposition of corrosion products. Indeed, each elementary emitting surface of the wall illuminates as well as the surface facing the detector without substantially limiting the radiation from the source S1 which is presented as an almost parallel beam at the

of the detector.

  Under these conditions, it can be said that the counting rate corresponding to the deposit is small compared to that which is

due to the source.

  The contribution of primary water activity to the detectors must also be taken into account. The corresponding count rate would be very low for the primary water activities corresponding to normal operation of the reactor. Since the count rate of the detector is a function of the number of water molecules encountered in a straight line by the radiation, this measured rate gives an image of the density of the

water at the detector.

  In particular, it is possible by this method to detect very particular two-phase configurations such as those of FIG. 6. In the central part there are vapor bubbles B whereas the peripheral zone P is

  entirely made up of liquid water.

  FIGS. 4 and 4 give, on the one hand, a marking of the water levels N1, N2... N8 (FIG. 4) with respect to the gamma sources S1 and S2 and, on the other hand, (FIG. 5). the rate of

  counting T (<s) according to the dimension z (in cm) for differences

  levels that are marked 1, 2 ... 8.

  The previously described method may be more difficult to implement in case of excessive activity of the primary water. To overcome this difficulty, we can use one or two detectors placed in such a way that they are not

  subject to radiation emitted by the sources. We can thus corri-

  in the total count rate the part that is due to

the activity of water.

  From a practical point of view, setting up a 60Co source of 100OCi is problematic because of the importance of the radiation protection it requires. It takes a 21cm lead container with a weight of about 1 ton. The contact dose is then less than

mrem / h.

  Therefore, it may be interesting to replace the gamma radiation with a neutron beam. One can in particular use a deuterium-tritium source giving 14 MeV neutrons with a emission of the order

  of 5.1010 s-1 neutrons in a solid angle of 4u.

  For a detector having the following characteristics: - surface. detection: 160 cm2 - efficiency: 0.01 one would have with a source and a detector on the same diameter, both being placed 30 cm from the wall, the orders of magnitude of the following counting rates T 100 o 1 in phase vapor, T 1.4 x 10 3 s 1 in liquid phase, -2 -. T 3,6 x 10 2 S 1for a diphasic mixture where 3/4 of

courses are in cash.

  In this case, we use of course a detector

  suitable for neutron detection.

  The exploitation of the counts is done like.with the gamma radiation and one can thus draw up a "map"

  axial density and therefore distributions liquid water-

steam water.

  It should be noted, however, that in the case where a neutron source is used, the counting rates indicated above show that it is practically impossible to detect

  the presence of a small proportion of vapor in the liquid.

  It can thus be seen that the method which is the subject of the invention makes it possible to draw up an axial reading of the density of the fluid filling the chamber and thus to deduce the level of liquid

  in the enclosure with a precision that depends on the number of

  used. It is also seen that it is possible to overcome the difficulties resulting from the possible activity of the water contained in the enclosure and deposits of corrosion products

on the inner wall of the enclosure.

Claims (10)

  1. A method for measuring the level of the surface of a liquid in an enclosure with a vertical axis, characterized in that placed on the same vertical near the wall of said enclosure N radiation sources capable of producing beams of radiation in said enclosure, in that placed on a second vertical contained in the plane of said axis and said first vertical N 'radiation detectors (N'> N) disposed at different levels, in that measured with said detectors the intensity of the radiation having passed through said enclosure, and in that the various measurements are deduced from the level of the surface   of the liquid and / or the local density of the diphasic mixture.   2. Method according to claim 1, characterized in   said radiation is y-radiation or neutrons.   3. Method according to claim 2, characterized in   what said neutrons emitted are fast neutrons.   4. Method according to any one of the claims   1 to 3, characterized in that there is at least one detector in a zone not reached by said radiation emitted by the sources for measuring the activity corresponding to the deposition of the active corrosion product on the wall of the enclosure and the own activity   said liquid contained in the enclosure.   5. Device for measuring the level of the surface of a liquid in an enclosure with a vertical axis, characterized in that it comprises:   - N sources of radiation arranged on the same first ver-   close to the wall of the enclosure, said sources being able to emit radiation passing through said enclosure, - N 'radiation detectors (N'> N) arranged at successive levels on a second vertical contained in the plane defined by the first vertical and said axis, said detectors being able to detect the radiation having passed through said enclosure, each detector measuring the intensity of the radiation received.   means for processing said measurements and deducing therefrom the level of the liquid surface and / or the local density of the diphasic mixture. l - to 2464462   6, Device according to claim 5, characterized   in that said sources are 60Co sources and said detectors are scintillators or chambers   of ionization sensitive to radiation y.   7. Device according to claim 5, characterized in that said sources are sources of neutrons and   in that said detectors are neutron detectors.   8. Device according to claim 7, characterized in that the neutron sources are of the reaction type   deuterium-tritium, reaction (a - n) or spontaneous fission.   9. Device according to any one of the   5 to 8, characterized in that the enclosure is a pressuri-   for a light water nuclear reactor in which the liquid water is in equilibrium with its vapor, the water being capable of being radioactive and the wall of the enclosure being capable of receiving deposits of radiological corrosion products, and said device further comprises at least one additional detector disposed outside the zone reached by the radiation, said detector being able to measure the   radiation due to the radioactivity of the water and said deposits.   10. Device according to any one of the   5 to 9, characterized in that it comprises between the wall of the enclosure and each of said detectors a device for   collimation of the radiation having passed through the enclosure. This provision   tif is formed of a set of parallel metal plates whose function is to reduce the contribution, at the rate of   counting the transmitters of parasitic radiation located at   proximity of the detectors (corrosion products deposited on the wall;   emitters dispersed in the volume of the pressurizer).